Analog amplifiers, in particular operational amplifiers and differential amplifiers are used in signal-processing units in which an output signal of, e.g. a temperature-, humidity-, force-, or pressure sensor is transformed and processed. In most cases, sensors and signal-processing units are combined into so-called measuring devices and are used in industrial environments. An important group of measuring devices which impose very stringent requirements on a signal-processing unit are the force-measuring devices, in particular the gravimetric measuring devices.
The gravimetric measuring devices such as thermo-analysis instruments, balances, humidity-measuring instruments and the like incorporate force-measuring cells which are based on different designs depending on the requirements imposed on their accuracy or measurement resolution. A representative example for a relatively economical design is the force-measuring cell with a deformable body and strain gauges.
As a rule, strain-gauge-based force measuring cells and measuring devices function according to the principle of resistance measurements. They make use of the phenomenon that the resistance of a strain gauge is proportionate to the strain, i.e., the relative change in length, and the length change, in turn, is proportionate to the force acting on the force-measuring cell. To perform an accurate measurement, the strain gauges are often connected in a bridge circuit.
The bridge circuit, as well as the half-bridge circuit, belongs to the known state of the art as a circuit arrangement in which at least one of the resistors used in the bridge circuit is variable in regard to its resistance value. This condition is met by many of the known sensors. For example, the electrical resistance of strain gauges changes dependent on the strain that they are subjected to.
For cost reasons and for the sake of simplicity, one chooses in general a DC power supply for the measuring device and thus also a DC amplification although, due to the commonly known technical limitations, this choice often has disadvantages, specifically DC voltage errors such as thermo-voltage errors or voltage-offset errors in the analog amplifier as well as excessive low-frequency noise in the entire circuit. Thermo-voltages can occur at the connecting joints of the required cables leading from the resistor bridge to analog amplifiers, if different materials and temperatures are involved at the connecting joints. These errors need to be held below an acceptable tolerance limit or, in other words, the measuring signal needs to stand out clearly from the error voltages.
The aforementioned measurement errors, such as thermo-electrical effects, noise and temperature effects in analog amplifiers that follow in the circuit chain are often reduced by the application of additional AC-modulation and -amplification techniques. This technique of modulation and amplification is also disclosed in EP 0 760 936 B2. A carrier-frequency generator produces an AC voltage signal (square-wave AC voltage signal) which is supplied to a bridge circuit. The output signal of the bridge is passed on by way of a symmetric differential amplifier to a demodulator. The demodulator in the form of a changeover switch is controlled by the carrier-frequency generator.
Analog amplifier circuits with switched amplifiers are used in many applications. In this arrangement, the two signals arriving from the measuring bridge circuit are each delivered to one operational amplifier during one measurement phase by means of two analog changeover switches. During a second measurement phase, however, the signal-connections are crossed over so that each signal is fed, respectively, to the other amplifier. This alternating changeover of the sensor signals is reversed again, for example with two further analog changeover switches which operate in phase with the input switches, putting the two amplified sensor signals alternatingly on direct and crossed-over paths.
These concepts offer many advantages. First, the offset voltage drift of the operational amplifiers can be suppressed. In a non-switched differential amplifier, after the two output signals of the differential amplifier have been subtracted from each other, the output signal also contains, in addition to the amplified sensor signal, the offset voltage difference of the two operational amplifiers multiplied by the differential amplification factor. In a switched differential amplifier, the offset voltage of the first operational amplifier is added in one phase to the first sensor signal, while the offset voltage of the second operational amplifier is added to the second sensor signal. As the sensor signals during the second measurement phase are crossed over before they are fed to the operational amplifiers, the same sensor signals now receive the respective offset voltage of the other operational amplifier. Accordingly, the offset voltage difference of the switched amplifier changes its polarity in each measurement phase. If the measurement phases follow each other for example at a sufficiently high frequency, the polarity changes will be fast enough to allow their removal from the signal by means of a low-pass filter.
As a second advantage, the 1/f-noise can be suppressed. Every operational amplifier exhibits below a certain frequency threshold a significant increase in noise which follows approximately a 1/f-relationship. In many applications, for example in force-measuring devices with strain gauge force-measuring cells, only static or very slow-changing signals are of interest, for example in the range from 0 to 1 Hz. With a non-switched differential amplifier, the 1/f-noise which predominates at these low frequencies is superimposed on the measurement signal, whereby the quality of the amplified sensor signals is severely compromised. With a switched differential amplifier, the changeover switches at the input of the differential amplifier or of the sensor cause the static or low-frequency sensor signals to be folded in the frequency range with the switchover signal of a switch controller and, among other things, to be raised to the frequency of the switchover signals. If the latter is sufficiently greater than the 1/f frequency threshold, only the white noise of the operational amplifier, which is much smaller at this point in the frequency range, is superimposed on the folded sensor signal. White noise is known in the engineering sciences and natural sciences as a physical noise whose amplitude is constant over the power density spectrum. The power of a random signal is obtained by integrating its power density spectrum over the entire resistance (from minus infinity to plus infinity). Thus, the white noise in a theoretical sense has an infinite signal energy. In practical cases, however, the power density of white noise falls off at very high frequencies.
With the second changeover switches at the output of the differential amplifier, the sensor signals are with regard to their frequency characteristics folded back again into the domain of DC or low frequency. In addition, the 1/f-noise of the operational amplifiers is folded upwards to the switchover frequency by the second changeover switches. This noise can subsequently be suppressed by means of a low-pass filter.
The concepts explained above keep their principal validity for a modulation of any desired shape. The sensor signals can for example be sinusoidally modulated at the input to the differential amplifier. At the output of the differential amplifier, the signals can subsequently be restored by means of sinusoidally controlled demodulators.
A sinusoidal signal is chosen in a case where, e.g. an AC voltage source is available or if the preservation of the signal over long connecting distances is of primary concern. In practice, the modulation with square-wave signals has established itself, due to the lower technical requirements for the circuit design which leads to lower costs. This type of modulation is realized often by means of analog switches.
With a square-wave AC modulation, for example by means of analog switches, the amplifier input signals change instantaneously. However, since analog amplifiers have only a limited bandwidth and rise time, their output signals cannot immediately follow the input signals. If the rise time is the limiting element, the amplifier output signal will ramp up approximately as a linear function of time. If the bandwidth is the limiting factor, the amplifier output signal will go through a transient oscillation with an approximately exponential decay. In any event, the assumption that the amplifier output signal equals the amplifier input signal multiplied by the amplification factor is no longer true during the transient oscillation. However, the assumption is based on a constant amplification factor. As long as the bandwidth and the rise time remain constant, the transfer errors caused by them in the amplifier output signal can be considered as constant. These transfer errors can for example be built into a scale factor as a fixed quantity and will thus disappear.
New, highly homogeneous materials for the deformable bodies of force-measuring cells, advanced manufacturing methods as well as precision strain gauges have made it possible to manufacture high-resolution force-measuring cells also based on strain gauge technology. In order to take advantage of the performance of these high-resolution force measuring cells, the signal-processing unit has to be adapted to meet the new requirements.
Extensive serial investigations of different analog amplifiers of the same build led to the conclusion that the bandwidth and rise time of analog amplifiers show in some cases a wide scatter of the results from one unit to the next and in addition that they behave differently under comparable temperature conditions (see FIG. 11). The term “scatter between units” in this context means the deviations in the behavior of elements of the same build relative to each other. The investigations have thus led to the conclusion that the speed of the transient decay, and therefore also the resultant error in amplitude, are variable. Thus, the scale factor, too, is likewise variable and is likewise subject to the scatter between units and the effects of temperature. These transfer errors have to be removed by performing an individual balancing of the scatter between units and compensation of the temperature effects. Procedures of this kind add considerably to the cost and make the manufacturing process as well as the product more expensive.